Most of today's electrical energy is generated by utilizing a thermodynamic cycle for creating mechanical work. The Carnot cycle is a theoretical thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot. This theoretical cycle sets an upper limit for the efficiency of any thermodynamic cycle for converting a given amount of heat into work between two thermal reservoirs. The ideal cycle for two-phase working fluids is the Rankine cycle. William J. M. Rankine provided the fundamental thermodynamic underpinning of the steam engine that is considered the practical Carnot cycle for a two-phase working fluid because the T-s diagram resembles the Carnot cycle. The main difference is that heat addition (in the boiler) and rejection (in the condenser) are isobaric in the Rankine cycle and isothermal in the theoretical Carnot cycle. A pump pressurizes the working fluid received from the condenser. All of the energy in pumping the working fluid through the cycle is lost, as is all of the energy of vaporization in the boiler which is rejected in the condenser. Pumping the liquid working fluid requires about 1-3% of the turbine power, much less than compressing a gas. The efficiency of a Rankine cycle is limited by the working fluid and equipment materials. Steam entry temperatures into the turbine are ˜565° C. and condenser temperatures are ˜30° C. This gives a theoretical Carnot efficiency of ˜63% and an actual efficiency of 42% for a modern power station. While many working fluids can be used, water is the fluid of choice since it is nontoxic, unreactive, abundant, low cost, and has good thermodynamic properties. When a Rankine cycle is implemented with organic working fluids, it is commonly referred to as on Organic Rankine cycle (ORC).
The classical Rankine engines have four discrete components: the boiler, the expander, the condenser and the pump and additionally involves a phase change between gas phase and liquid phase. In a classical Rankine cycle that runs at a maximal temperature given by the material properties of the expansion device, a part of losses is associated with the boiler due to conductive and convective exergetic losses and due to inherent losses associated with a pool boiling process. With the current trend to avoid exergetic losses of low grade heat and to collect low grade heat as part of solar technologies there is a growing demand for low temperature conversion engines. This area is sometimes covered by ORC engines because at lower pressures and temperatures the steam cycle requires too large expansion devices while organic fluids can maintain the same device size ratios as it was originally established for higher temperature steam Rankine cycles. Both steam and organic Rankine engines have low exergetic efficiencies compared to the upper limit given by the Carnot particularly at low temperatures.